The long-temri objective of this research project is to define the molecular basis of polycythemia vera (PV). PV represents the ultimate phenotype of the mutant kinase, JAK2 V617F, through its effects on committed hematopoietic progenitor cells but we hypothesize that while JAK2 V617F expression is responsible for the clinical phenotype of PV, the underlying molecular mechanisms responsible for the disease reside in the hematopoietic stem cell (HSC), which does not require JAK2, and that the thrombopoietin (TPO) receptor, Mpl, is integrally Involved In these mechanisms. This hypothesis, which is based on our discovery of impaired Mpl expression in PV with an attendant increase in plasma TPO, and our observation that absence of the MPL gene abrogated the PV phenotype in a JAK2 V617F transgenic (tg) mouse model of PV, provides a mechanistic basis for understanding the pathophysiology of PV at the stem cell level and a rational approach to therapy To this end, we propose to use genetic techniques to dissect the influence of the MPL:TPO axis, and also the roles of specific genes up regulated in human PV CD34+ cells, on the behavior of HSC in the murine JAK2 V617F tg model of PV. In Specific Aim 1, we will examine the effect of abrogation of the TPO gene on the phenotype of the JAK2 V617F tg mouse, on the size of its HSC pool and the HSC gene expression profile, by breeding with a TPO-/- mouse. Control experiments will employ mice in which Mpl function was abrogated by a point mutation or a gene knockout independent of MPL that impairs platelet production, while a neutralizing Mpl antiserum will be examined as a model of targeted therapy. In Specific Aim 2, we will create a tg mouse expressing a PV Mpl splice variant and assess its phenotype in the presence and absence of JAK2 V617F. We also examine by crossbreeding, the effect of knocking out SPARC or LCN2, two genes that are up regulated in PV, on the phenotype of the JAK2 V617F tg mouse. Finally, in Specific Aim 3, we will use xenotranslantation in NOG mice to examine the in vivo behavior of genetically-defined PV CD34 + cells from clinically distinct PV patient populations that we have identified by gene expression profiling and unsuoervised hierarchical clustering.